use `respond_to?` instead of `method_defined?` when aliasing `search`
use `respond_to?` instead of `method_defined?` when aliasing `search`

Fixes #305

guilleva/ransack,AndrewSwerlick/ransack,mariorcardoso/ransack,Mario245/ransack,vaski/ransack,martndemus/ransack,activerecord-hackery/ransack,outstand/ransack,laserlemon/ransack,praktijkindex/ransack,roman-franko/ransack,nagyt234/ransack,amboxer21/ransack,avit/ransack,Eric-Guo/ransack,reinteractive/ransack,johan--/ransack